(Also known as serine/threonine kinase 11, STK11) tumor suppressor is mutationally inactivated in Peutz-Jeghers syndrome (PJS), an autosomal dominant disorder characterized by gastrointestinal polyps, mucocutaneous pigmentation, and a markedly increased risk for malignant tumors. Inactivating mutations in LKB1 are also found in cancer patients without PJS, including sporadic lung adenocarcinoma (common), ovarian cancer, breast cancer, pancreatic cancer, and biliary adenocarcinoma (rare). Although LKB1 encodes a protein kinase to which a variety of functions have been ascribed, as yet no unifying hypothesis has convincingly explained how loss of LKB1 function contributes to carcinogenesis. Therefore, the long-term goal of this project is to identify the molecular mechanism underlying the tumor suppressor function of LKB1, a prerequisite for targeting LKB1 function in cancer treatment. In the last decade, considerable evidence has accumulated suggesting that genomic instability plays an important role in tumorigenesis. Supporting a role in tumor suppression, our preliminary findings suggest that LKB1 plays a critical role in maintaining genomic stability. We found that LKB1 interacts with ATM, BRCA1, and other DNA damage response (DDR) proteins in vivo. LKB1-deficient cells exhibit a delayed DDR and reduced DNA repair after exposure to ionizing radiation (IR). LKB1 deficiency enhances intracellular reactive oxygen species (ROS) accumulation, which is a major cause of DNA damage and genetic instability. Ectopic expression of LKB1 reduces intracellular ROS levels. Based on these observations, we hypothesize that LKB1 suppresses tumorigenesis by safeguarding genomic stability through regulating DNA damage/repair response and inhibiting intracellular ROS production. We have designed the following specific aims to test our hypothesis. (1) Determine the role of LKB1 in IR- induced DNA damage/repair response. Hypothesis to be tested: LKB1 is a DDR protein that regulates the enzymes involved in DNA-damage repair and DNA replication. As a corollary, loss of LKB1 may lead to an accumulation of DNA damage, an increase in genomic instability, and augmented tumorigenesis. (2) Determine the role of ROS in the function of LKB1. Hypothesis to be tested: LKB1 protects the genome from ROS-induced oxidative stress by regulating antioxidant gene production. (3) Determine the protein signatures and signaling networks in IR- and H2O2- treated cells in the presence or absence of LKB1 and identify novel LKB1-interacting proteins under stressed conditions. Hypothesis to be tested: LKB1 interacts with specific proteins to mediate its role in the maintenance of genomic stability. Significance. Accomplishment of these objectives would suggest that LKB1 integrates different signals (DNA damage response, DNA repair, and antioxidant) and interacts with specific proteins in the maintenance of genomic stability. The findings should lead to a better understanding of the role that LKB1 mutation may play in the pathogenesis of PJS and tumorigenesis. These studies will also provide rationale for pharmacologic replacement or mimicry of LKB1 function in cancer prevention and treatment. PUBLIC HEALTH RELEVANCE: LKB1 is a serine/threonine kinase that is mutationally inactivated in Peutz-Jeghers syndrome and many cancers. It is estimated that 93% of PJS patients will develop a malignant tumor at an average age of 43 years 3. Also, more than a third of sporadic lung adenocarcinomas have been reported to have LKB1 mutations 5, 6. Although a variety of functions have been ascribed to LKB1, as yet no individual hypothesis has convincingly explained how loss of LKB1 function contributes to carcinogenesis. Thus, the long-term objective of this project is to identify the molecular mechanism underlying LKB1's tumor suppressor function, a prerequisite for the development of pharmacological agents that can modulate or mimic LKB1 function for cancer prevention and treatment. Based on our preliminary studies, we hypothesize that LKB1 suppresses tumorigenesis by safeguarding genomic stability through its regulation of the DNA damage/repair response and inhibition of intracellular ROS generation. In light of the high incidence of LKB1 mutations in primary human tumors of diverse origin, understanding the functional roles of LKB1 in genome stability will provide valuable new information to further our current knowledge of carcinogenesis and cancer therapy. Thus, the findings from these studies will be widely applicable.